[PATCH v2] jbd2: fix the inconsistency between checksum and data in memory for journal sb

[Ye Bin <yebin@huaweicloud.com>]

From: Ye Bin <yebin10@huawei.com>

Copying the file system while it is mounted as read-only results in
a mount failure:
[~]# mkfs.ext4 -F /dev/sdc
[~]# mount /dev/sdc -o ro /mnt/test
[~]# dd if=/dev/sdc of=/dev/sda bs=1M
[~]# mount /dev/sda /mnt/test1
[ 1094.849826] JBD2: journal checksum error
[ 1094.850927] EXT4-fs (sda): Could not load journal inode
mount: mount /dev/sda on /mnt/test1 failed: Bad message

The process described above is just an abstracted way I came up with to
reproduce the issue. In the actual scenario, the file system was mounted
read-only and then copied while it was still mounted. It was found that
the mount operation failed. The user intended to verify the data or use
it as a backup, and this action was performed during a version upgrade.
Above issue may happen as follows:
ext4_fill_super
 set_journal_csum_feature_set(sb)
  if (ext4_has_metadata_csum(sb))
   incompat = JBD2_FEATURE_INCOMPAT_CSUM_V3;
  if (test_opt(sb, JOURNAL_CHECKSUM)
   jbd2_journal_set_features(sbi->s_journal, compat, 0, incompat);
    lock_buffer(journal->j_sb_buffer);
    sb->s_feature_incompat  |= cpu_to_be32(incompat);
    //The data in the journal sb was modified, but the checksum was not
      updated, so the data remaining in memory has a mismatch between the
      data and the checksum.
    unlock_buffer(journal->j_sb_buffer);

In this case, the journal sb copied over is in a state where the checksum
and data are inconsistent, so mounting fails.
To solve the above issue, update the checksum in memory after modifying
the journal sb.

Fixes: 4fd5ea43bc11 ("jbd2: checksum journal superblock")
Signed-off-by: Ye Bin <yebin10@huawei.com>
---
 fs/jbd2/journal.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index d480b94117cd..bf255f8b5eeb 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -2349,6 +2349,12 @@ int jbd2_journal_set_features(journal_t *journal, unsigned long compat,
 	sb->s_feature_compat    |= cpu_to_be32(compat);
 	sb->s_feature_ro_compat |= cpu_to_be32(ro);
 	sb->s_feature_incompat  |= cpu_to_be32(incompat);
+	/*
+	 * Update the checksum now so that it is valid even for read-only
+	 * filesystems where jbd2_write_superblock() doesn't get called.
+	 */
+	if (jbd2_journal_has_csum_v2or3(journal))
+		sb->s_checksum = jbd2_superblock_csum(sb);
 	unlock_buffer(journal->j_sb_buffer);
 	jbd2_journal_init_transaction_limits(journal);
 
@@ -2378,9 +2384,17 @@ void jbd2_journal_clear_features(journal_t *journal, unsigned long compat,
 
 	sb = journal->j_superblock;
 
+	lock_buffer(journal->j_sb_buffer);
 	sb->s_feature_compat    &= ~cpu_to_be32(compat);
 	sb->s_feature_ro_compat &= ~cpu_to_be32(ro);
 	sb->s_feature_incompat  &= ~cpu_to_be32(incompat);
+	/*
+	 * Update the checksum now so that it is valid even for read-only
+	 * filesystems where jbd2_write_superblock() doesn't get called.
+	 */
+	if (jbd2_journal_has_csum_v2or3(journal))
+		sb->s_checksum = jbd2_superblock_csum(sb);
+	unlock_buffer(journal->j_sb_buffer);
 	jbd2_journal_init_transaction_limits(journal);
 }
 EXPORT_SYMBOL(jbd2_journal_clear_features);
-- 
2.34.1